Proces for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes

Abstract: A process for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes of formula wherein R and R₁ have the meanings reported in the description, comprising the transformation of a 2,2-dialkyl-1,3-dioxan-5-carboxylic acid ester of formula wherein R₂ has the meanings reported in the description into the corresponding amide and the subsequent rearrangement of this latter into the compounds of formula I, is described.

Description: The present invention relates to a process for the preparation of ketals of 2-amino-1,3-propanediol and more particularly relates to a process for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes. The ketals of 2-amino-1,3-propanediol are advantageously used as synthetic intermediates in the preparation of the compound (S)-N,N'-bis-[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-hydroxy-1-oxopropyl)amino]-2,4,6-triiodo-1,3-benzenedicarboxamide, known with the International Nonproprietary Name of Iopamidol (Merck Index, XI ed., page 799, No. 4943). Iopamidol was first described by the Swiss Company Savac A.G. in the British patent No. 1,472,050 and is used in diagnostics as a non ionic X-rays contrast medium. The preparation of Iopamidol, described in said patent, comprises the condensation reaction of L-5-(2-acetoxy-propionylamino)-2,4,6-triiodo-isophthalic acid dichloride with 2-amino-1,3-propanediol, better known as serinol, in dimethylacetamide and in the presence of a base. Alternatively, in the same patent, it is described a method comprising the condensation reaction of the above acid dichloride, with a ketal of serinol; the subsequent acid hydrolysis of the formed diketal, carried out by conventional techniques, allows thus to obtain the desired product. Among the possible ketals of serinol which can be used in said synthesis, for instance, 5-amino-2,2-dialkyl-1,3-dioxanes are cited. Several processes for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes are reported in the literature. The British patent application No. 2,081,256 (Rhône-Poulenc Industries) and the United States patent No. 3,812,186 (Eprova A.G.) describe the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes by catalytic hydrogenation of the corresponding 5-nitro derivatives, prepared, in turn, by direct cyclization of 2-nitro-1,3-propanediol with a suitable ketone, in the presence of boron trifluoride etherate. In the United States patent No. 4,978,793 (W.R. Grace & Co.) is described the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes which comprises at first the synthesis of the corresponding 5-nitro-derivatives, through a three step process starting from nitromethane and formaldehyde and subsequently the reduction of the nitro group. The processes for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes described in the literature evidence the remarkable drawback of using nitro derivatives as intermediates, which are particularly unstable and explosive compounds. Now we have found and it is the object of the present invention, a process for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes of formula wherein R and R 1 , the same or different, represent a straight or branched C 1 -C 3 alkyl or, together with the carbon atom to which they are bonded, form a C 5 -C 6 cycloaliphatic ring; comprising the transformation carried out by treatment with ammonia of a 2,2-dialkyl-1,3-dioxane-5-carboxylic acid ester of formula wherein R 2 represents a straight or branched C 1 -C 3 alkyl group, a phenyl optionally substituted by nitro groups or a benzyl; into the corresponding amide of formula wherein R and R 1 have the above reported meanings and the subsequent rearrangement to obtain 5-amino-2,2-dialkyl-1,3-dioxanes of formula (I). The compounds of formula (I) thus obtained can be used as such, for instance as described in the aforementioned British patent No. 1,472,050, or hydrolyzed according to conventional techniques to obtain serinol. Specific examples of the compounds of formula (I) are: 5-amino-2,2-dimethyl-1,3-dioxane 5-amino-2,2-diethyl-1,3-dioxane 5-amino-2,2-di-n.propyl-1,3-dioxane 5-amino-2-ethyl-2-methyl-1,3-dioxane 3-amino-1,5-dioxaspiro[5.5]undecane 3-amino-1,5-dioxaspiro[4.5]decane The transformation of the ester of formula II into the corresponding amide of formula III is carried out by treatment with ammonia, preferably in an aqueous solution. The compounds of formula III are then easily isolated by evaporating the solvent. The rearrangement of the intermediate amides of formula III is carried out by treatment with a hypohalogenite such as, for instance, sodium hypobromite or sodium hypochlorite, in an aqueous solution. Preferably sodium hypobromite, optionally prepared in situ from sodium hydroxide and bromine, is used. The compounds of formula II are known or easily prepared according to known methods (H.A. Bates et al., J. Org. Chem., 1986, 51 , 2637-2641). The 5-aminocarbonyl-2,2-dialkyl-1,3-dioxanes of formula III are new and they are a further object of the present invention. The preparation of the amides, in general, can be carried out according to several methods described in the literature (see for a general review March, Advanced Organic Chemistry, IV ed., pages 1275-1276). In this connection, without exceed the scope of the present invention, 5-aminocarbonyl-2,2-dialkyl-1,3-dioxanes of formula III, can be thus prepared according to the aforementioned methods. A preferred embodiment of the process object of the present invention is the following. A 2,2-dimethyl-1,3-dioxane-5-carboxylic acid ester is treated with an aqueous solution of ammonia for a few hours at room temperature. By evaporating the solvent the crude 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane is obtained in practical quantitative yields and used as such in the subsequent rearrangement reaction. 5-Aminocarbonyl-2,2-dimethyl-1,3-dioxane is added to a basic aqueous solution of sodium hypobromite, prepared in situ from sodium hydroxide and bromine, and heated for a few hours at 80°C. After the evaporation of the water, the residue is collected and purified by treatment with an organic solvent affording 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane in good yields. The process object of the present invention is particularly suitable for an industrial application also because it uses starting products easy to be prepared. Both the intermediates and the starting products are stable and therefore do not present the risk of dangerous and unwanted reactions (explosions) during the accomplishment of the process. Moreover the process object of the present invention does not require drastic reaction conditions, such as for instance reduction at high pressures. Another particularly advantageous aspect is that the whole process is carried out in an aqueous environment allowing an easy recovery of the desired products in good yields. With the aim to better illustrate the present invention, without limiting it, the following examples are now given. Example 1 Preparation of 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane A mixture of 2,2-dimethyl-5-ethoxycarbonyl-1,3-dioxane (4.1 g; 21.8 mmoles) and aqueous ammonia at 30% (10 ml) was kept under vigorous stirring for 20 hours. At the end of the reaction the water was evaporated at reduced pressure; 3.4 g of crude 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane (98% yield) were obtained, which could be used as such in the subsequent rearrangement reaction. By crystallization from toluene 2.76 g of pure product (80% yield) were obtained. 1 H-NMR (DMSO-d 6 ), δ (ppm), J (Hz): 1.27 (3H, s); 1.35 (3H, s); 2.6 (1H, m); 3.85 (4H, m); 7.0 (1H, bs); 7.3 (1H, bs). 13 C-NMR (DMSO-d 6 ), δ (ppm): 19.9 (q); 27.7 (q); 40.6 (d); 60.8 (t); 97.1 (s); 172.11 (s). IR (KBr): cm -1 3360, 3200, 1665, 1635 Mass (m/e): 160 (M+1) + Example 2 Preparation of 5-amino-2,2-dimethyl-1,3-dioxane Bromine (6 g; 37.5 mmoles) was added dropwise in 5 minutes and keeping the temperature between 0°C and 5°C, to a solution of sodium hydroxide (4.5 g; 112.5 mmoles) in water (22 ml) cooled at 0°C. The mixture was kept under stirring at 0°C for 15 minutes and 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane (3 g; 18.75 mmoles) was added in one portion. The mixture was kept under stirring at 0°C for 1 hour; then it was heated at 80°C for 16 hours. At the end, water was evaporated under vacuum, the residue was collected with methylene chloride (25 ml), anhydrous sodium sulphate was added and stirring was maintained for 15 minutes. The inorganic salts were removed by filtration and the solvent was evaporated at reduced pressure. 1.4 g of crude 5-amino-2,2-dimethyl-1,3-dioxane containing 5% of the starting material were obtained. The pure product was obtained by distillation. Example 3 Preparation of 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane A mixture of 2,2-dimethyl-5-methoxycarbonyl-1,3-dioxane and 2,2-dimethyl-5-ethoxycarbonyl-1,3-dioxane having a GC titre of 77% and 18%, respectively (850 g; 3.76 moles and 0.814 moles, respectively) and aqueous ammonia at 30% (940 ml) was kept under vigorous stirring at room temperature and under nitrogen atmosphere for 24 hours. At the end, the reaction mixture was cooled at 0°C and kept under stirring for 60 minutes. A solid was obtained which was filtered, washed with water (100 ml) and dried under vacuum at 70°C affording 5-aminocarbonyl-2,2-dimeth yl-1,3-dioxane (541 g; 74% yield) which was used as such in the subsequent rearrangement reaction. m.p. 132-134°C. Example 4 Preparation of 5-amino-2,2-dimethyl-1,3-dioxane Bromine (6 g; 37.5 mmoles) was added dropwise in 5 minutes and keeping the temperature between 0°C and 5°C, to a solution of sodium hydroxide (4.5 g; 112.5 mmoles) in water (22 ml) cooled at 0°C. The mixture was kept under stirring at 0°C for 15 minutes; the solution was added to 5-aminocarbonyl-2,2-dimethyl-1,3-dioxane (3 g; 18.75 mmoles). It was kept under stirring at 0°C for 1 hour; then it was heated at 80°C for 16 hours. At the end, water was evaporated under vacuum, the residue was collected with methylene chloride (25 ml), anhydrous sodium sulphate was added and stirring was maintained for 15 minutes. The inorganic salts were removed by filtration and the solvent was evaporated at reduced pressure. 1.4 g of crude 5-amino-2,2-dimethyl-1,3-dioxane containing 5% of the starting material were obtained. The pure product was obtained by distillation. Example 5 Preparation of 2-amino-1,3-propanediol hydrochloride A) A solution of sodium hydroxide at 30% in water (122 ml) and a commercial solution of sodium hypochlorite 1.85 M (250 ml) were charged into a 600 ml reactor provided with mechanical stirrer, at 15°C and under nitrogen atmosphere. 5-Aminocarbonyl-2,2-dimethyl-1,3-dioxane (60 g; 0.38 moles), prepared as described in example 3, was therein added and the suspension was kept at 15°C under stirring for one hour. The reaction mixture was gradually heated up to 70°C in a period of 2 hours and kept under stirring at this temperature for 90 minutes. At the end, the reaction mixture was distilled and 438 g of aqueous solution which, by titration with HCl 0.1 N resulted to contain 5-amino-2,2-dimethyl-1,3-dioxane (10.37% titre; 0.346 moles; 91.2% yield), were collected. Such a solution was used as such in the following step. B) HCl at 37% (33 ml) was added portionwise, in 30 minutes and at 25°C, to the aqueous solution prepared as above described (A). The reaction mixture was kept under stirring at 40°C for 60 minutes. By evaporating the mixture under vacuum an oily residue, which was kept under stirring at 40°C, was obtained. The crude product was crystallized from acetone (100 ml) and the solid obtained was filtered and washed with acetone (10 ml). After drying under vacuum at 40°C, 34.5 g of 2-amino-1,3-propanediol hydrochloride (74% yield; GC titre 99%) were thus obtained. The serinol hydrochloride could be converted into the corresponding free base by conventional techniques such as, for instance, by using ammonia or alkaline solutions.

Claims: A process for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes of formula wherein R and R 1 , the same or different, represent a straight or branched C 1 -C 3 alkyl or, together with the carbon atom to which they are bonded, form a C 5 -C 6 cycloaliphatic ring; comprising the transformation carried out by treatment with ammonia of a 2,2-dialkyl-1,3-dioxane-5- carboxylic acid ester of formula wherein R 2 represents a straight or branched C 1 -C 3 alkyl group, a phenyl optionally substituted by nitro groups or a benzyl; into the corresponding amide of formula wherein R and R 1 have the above reported meanings and the subsequent rearrangement to obtain 5-amino-2,2-dialkyl-1,3-dioxanes of formula (I). A process according to claim 1 wherein the transformation of the ester of formula II into the corresponding amide of formula III is carried out by treatment with ammonia in an aqueous solution. A process according to claim 1 wherein the rearrangement is carried out by treatment with a hypohalogenite in an aqueous solution. A process according to claim 3 wherein the hypohalogenite is sodium hypobromite. A process according to claim 1 for the preparation of 5-amino-2,2-dimethyl-1,3-dioxane. A compound of formula wherein R and R 1 , the same or different, represent a straight or branched C 1 -C 3 alkyl or, together with the carbon atom to which they are bonded, form a C 5 -C 6 cycloaliphatic ring. 5-Aminocarbonyl-2,2-dimethyl-1,3-dioxane. A process for the preparation of 5-amino-2,2-dialkyl-1,3-dioxanes of formula wherein R and R 1 , the same or different, represent a straight or branched C 1 -C 3 alkyl or, together with the carbon atom to which they are bonded, form a C 5 -C 6 cycloaliphatic ring; comprising the rearrangement of the amide of formula wherein R and R 1 have the above reported meanings by treatment with a hypohalogenite in an aqueous solution to obtain 5-amino-2,2-dialkyl-1,3-dioxanes of formula (I).

Inventors: NARDI ANTONIO, VILLA MARCO, NARDI, ANTONIO, VILLA, MARCO
Applicants: ZAMBON SPA, ZAMBON GROUP S.P.A.
Application year: 1994
Main classification: C07D319
IPC codes: C07D233, C07D319